High molecular weight linear polyesters and copolyesters are well known and have enjoyed continued and increasing commercial success. These are especially useful resins because they provide compositions with excellent moldability, and molded articles therefrom having smooth and glossy surface appearance, high strength, stiffness, temperature resistance and other desirable properties.
Additionally, these resins may be modified to improve certain physical properties and further broaden their scope of application by incorporating therein various additives including fillers and reinforcing agents to enhance toughness and vary the stiffness of the material. Further, such resins and modified resins can be blended with other thermoplastic materials to enhance various physical properties and improve processability.
However, many polyester resins and resin blends suffer from and their applications are limited by their high flammability. In particular, where such resins are to be desired for many fields of use such as in home, construction, automobile and aircraft manufacturing, packaging, electrical equipment and the like, it is most desirable and often required that such materials be resistant to or have low flammability.
It is difficult to render polybutylene terephthalate resins (hereinafter referred to alternatively as "PBT" resins) flame retardant without sacrificing, to some degree, their inherent superior physical properties. Specifically, the use of conventional flame retardant additives, in conventional amounts, generally causes marked decreases in some of the advantageous physical properties of the PBT resins.
The U.S. Pat. No. 4,278,591 teaches that the combination of an ammonium polyphosphonate and a polyphosphonate ester is useful as a flame retardant in certain polymers. However, the cited patent teaches that such a combination has not been found useful as a flame retardant in the polybutylene terephthalate polymers apparently due to the higher processing temperatures used with these polymers causing decomposition of the ammonium polyphosphonate.
It has therefore been unexpectedly discovered that a mixture of a polyphosphonate ester, an ammonium phosphate and a halogenated benzo-sulfonate is an effective flame retardant for polyester resins, including polybutylene terephthalate resins.